1. Field of the Invention
The invention relates to a light emitting device where light emitted from a light emitting diode (hereinafter referred to as LED) is absorbed and wavelength-converted by phosphor and then the wavelength-converted light with a different wavelength is radiated therefrom.
2. Description of the Related Art
Light emitting devices that allow light emitted from an LED element to be wavelength-converted by phosphor are known. For example, some of the light emitting devices are composed such that phosphors are contained in a sealing resin, such as epoxy resin, to seal the LED element.
By sealing the LED element with sealing resin, a high degree of freedom in designing the light emitting device and an enhanced productivity can be obtained. However, due to the light radiated from the LED element, the optical and chemical characteristics of sealing resin deteriorate and, thereby, the emission efficiency of light emitting device lowers.
Also, there is a problem that the emission efficiency of phosphor lowers. When the device is used under high humidity conditions, the epoxy resin absorbs moisture since it has a moisture absorption property. The moisture absorbed there into causes a deterioration or decomposition in phosphor. As a result, the emission efficiency of phosphor lowers.
Further, it is known that, due to intensive light emitted from the LED element, the epoxy resin becomes colored with time. When the resin is colored, part of light to be emitted from the LED element is absorbed by the resin and, thereby, the optical output of light emitting device lowers.
Japanese patent application laid-open No. 11-204838 (hereinafter referred to as prior art 1) discloses a light emitting device that an LED element is sealed with a glass layer which contains phosphor (See FIG. 1 described ibid.).
FIG. 1 is a cross sectional view showing the light emitting device disclosed in prior art 1.
The light emitting device 30 is composed of: leads 31, 32; a cup portion 33 formed in the lead 33; the LED element 34 bonded onto a bottom portion 33A in the cup portion 33; wires 35 that connects between the electrodes of LED element 34 and the leads 31, 32; a glass layer 36 that seals the LED element 34 filled in the cup portion 33; phosphor 36A contained in the glass layer 36; and transparent sealing resin 37 that is formed into a lamp shape and seals the entire device.
In this composition, since the glass layer 36 with no moisture absorption property is used in place of the epoxy resin, moisture is not penetrated through the glass layer 36 and, therefore, the phosphor does not deteriorate. Further, because of using no epoxy resin, the optical output of light emitting device does not lower.
Japanese patent application laid-open No. 2000-31547 (hereinafter referred to as prior art 2) discloses a light emitting device that multiple LED elements are arrayed like a matrix (plane matrix) and light to be emitted from the LED's is wavelength-converted to offer light with a desired color (See FIG. 2 described ibid.).
FIG. 2 is a cross sectional view showing the light emitting device disclosed in prior art 2.
The light emitting device 50 is composed of: a base 51; a reflection frame 52; recesses 53 provided in the reflection frame 52; a lead 54; LED elements 55; a wavelength conversion sheet 56 that includes a transparent sheet board 56A and a wavelength conversion material layer 56B; and a diffusion plate 57. The interior of recess 53 is filled with mold resin 58. The base 51, reflection frame 52, wavelength conversion sheet 56 and diffusion plate 57 are integrally sealed with the mold resin 58.
The multiple LED elements 55 arrayed in plane are mounted on the base 51, and electric power is supplied to the LED elements 55 through the lead 54 from a power source (not shown). The reflection frame 52 attached to the base 51 allows light emitted from the LED elements 55 to be reflected in the recess 53 and to be radiated upward.
The wavelength conversion sheet 56 is disposed over the reflection frame 52. The wavelength conversion sheet 56 is made such that the wavelength conversion material layer 56B prepared by uniformly mixing a wavelength conversion material into resin binder is coated on the transparent sheet board 56a of sheet resin film, glass etc. and is cured. The wavelength conversion material is excited by light to be emitted from the LED element 55 and radiates excited light. Light with a predetermined wavelength is generated when the excited light is mixed with light to be directly emitted from the LED element 55.
Since light to be emitted from the LED elements 55 arrayed in plane is wavelength-converted by the wavelength conversion sheet 56, the light emitting device 50 can offer a good wavelength conversion efficiency and an even emission color. Therefore, unevenness in emission color can be reduced.
However, the light emitting device in prior art 1 has problems described below.
(1) When the glass layer 36 is filled in the cup portion 33 to seal the LED element 34, the phosphor 36A is deposited collected at a region around the LED element 34. Thereby, emitted light is confined or absorbed by the phosphor-collected region and, therefore, the external radiation efficiency of LED element lowers significantly.
(2) It is very difficult to make the wavelength conversion characteristic even since the disposition of phosphor depends on the filling process of glass material. To prevent this, it is necessary to severely control the mixing state of phosphor to be contained in the glass layer 36.
On the other hand, the light emitting device in prior art 2 has problems described below.
(1) Since light emitted from the LED element 55 cannot be externally radiated without passing through the wavelength conversion material layer 56B, the external radiation efficiency of light emitting device lowers that much.
(2) Since the reflection frame 52, wavelength conversion sheet 56 and diffusion plate 57 are integrally sealed with the mold resin 58, the entire light emitting device must have an increased thickness.